Photoelectron spectra of the lanthanide trihalides and their interpretation

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Abstract
The He i photoelectron spectra of gaseous LaCl3, LaBr3, LaI3, CeBr3, CeI3, NdBr3, NdI3, ErI3, LuBr3, and LuI3 have been obtained. They display a pronounced increase in splitting, and hence a progressively clearer definition of peaks in the valence band as either the halogen or the lanthanide increases in atomic number. These experimental features, together with a refined relativistic Xα DVM calculation using the von Barth–Hedin potential, have enabled us to assign these peaks with confidence. The He ii photoelectron spectra of CeBr3, NdBr3, and LuI3 are also presented. They reveal that the 4f-like ionizations of early lanthanide members (e.g., Ce) occur at lower energy than the ligand valence band, but that those of late members (e.g., Lu) are corelike. The aforementioned calculations reproduce this behavior quantitatively. They also help to rationalize a bimodal behavior in the valence band; the spectra with less than half-filled 4f shell are very similar, as are those with more than half-filled 4f shell, but the two groups are distinctly different. The width of the valence bands, which varies over a factor 2.5, is correctly reproduced. The calculations have been been extended to include fluorides, where contact is made with electron impact mass spectrometry and x-ray photelectron spectra of solids, thereby enabling corrections to be made to the latter. The fragmentation behavior in mass spectrometry is then related to the states observed in photoelectron spectroscopy. Both the calculations and these experimental comparisons yield a picture in which the lanthanide fluorides display predominantly ionic bonding (Ln2.2+); the bonding takes on succesively more covalent character as one proceeds to chlorides (Ln1.6+), bromides (Ln1.3+), and iodides (Ln1.0+).

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